Polycrystalline silicon (p-Si) and amorphous silicon (a-Si) are often used as the active layer material for thin film transistors (TFTs) in liquid crystal display (LCD); devices. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, amorphous silicon (a-Si) is commonly used in liquid crystal displays (LCD). Unfortunately, amorphous silicon (a-Si) TFTs have relatively low carrier mobility that limit their suitability for large area LCD. In contrast, polycrystalline silicon TFTs provide much faster carrier mobility. Thus, polycrystalline silicon (p-Si) is well suited for use in large LCD devices.
One method of crystallizing amorphous silicon into polycrystalline silicon is sequential lateral solidification (SLS), which can be used to form a polycrystalline silicon film with carrier mobility better than that formed by conventional excimer laser annealing (ELA). Therefore, in the interest of enhanced carrier mobility, low temperature polycrystalline silicon thin film transistor (LTPS TFT) formed by SLS technology has been used commonly in the application of System-on-Panel (SOP).
As a polycrystalline silicon film is attained by SLS silicon crystallization, the poly-silicon grains have the same orientation. Thus, if the channel directions of the TFT devices of an LCD device formed on the SLS-crystallized poly-silicon film are different to each other, the number of silicon grain boundaries exist on the carrier paths in different TFT devices are different, such that the electrical characteristic of the LCD panel fabricated thereby is irregular. Therefore, it is general to design the TFT devices following a same channel direction. However, by doing so, the freedom of circuit design is limited.
Please refer to FIG. 1, which shows a polycrystalline silicon film 10 having silicon grains to be grown by SLS technology on a slant relative to the horizontal direction of a substrate with a predetermined angle degree, being disclosed in U.S. Pat. No. 6,521,473. As seen in FIG. 1, when the growth direction of silicon grains is on a slant relative to the horizontal direction of the substrate at an angle of about 45 degree, the number of grain boundaries inside the TFT device 12, in which the channel direction is horizontal to the substrate, is the same as that of the TFT device 12′, in which the channel direction is perpendicular to the substrate, the result is that the electrical characteristic of the TFT device 12 is the same as that of the TFT device 12′, and thus the electrical irregularity can be prevented. However, the TFT devices can have uniform carrier mobility among the devices only when they are oriented either horizontal or perpendicular to the substrate.
Therefore, different from the aforesaid concept of growing silicon grains following a predetermined angle degree by SLS technology, it is intended to provide a thin film transistor device with high symmetry, in which the symmetrical structure of the TFT devices is utilized to enable the number of silicon grain boundaries exist on the carrier paths in different TFT devices to be the same, thereby not only the freedom of circuit design is increased since the orientation of the TFT devices are not limited, but also the circuit area of a TFT device occupied is reduced.